A communication apparatus, including a facsimile apparatus and a telephone set, establishes a direct current (DC) loop by a relay. The relay is operated by, for example, providing a transmitting/receiving device in an off-hook state or a pressing a speakerphone key, which are performed by an operator.
In order to reduce the size of a communication apparatus such as a facsimile apparatus or a telephone set, a semiconductor data access arrangement (DAA) has been used in an interface (I/F) between the communication apparatus and a telephone line. The DAA performs a line connect operation instead of a relay. Therefore, in a communication apparatus having the DAA, when the transmitting/receiving device is provided at an off-hook state or the speakerphone key is pressed, the DAA starts to make a DC loop and establishes the DC loop. When a call signal is transmitted from the communication apparatus to a repeating installation, the repeating installation transmits a dial tone to the communication apparatus in response to the call signal. When a telephone number designating a called apparatus is inputted by an operator of the communication apparatus, dial signals corresponding to digits of the telephone number are transmitted to the repeating installation through the established line.
When the dial signals transmitted from the communication apparatus are received by the repeating installation, the transmission of the dial tone in the repeating installation is stopped. Simultaneously, a called apparatus corresponding to the telephone number is called by the repeating installation or another repeating installation. When the called apparatus responds to the call from the repeating installation, a communication state is established between the calling side communication apparatus and the called side communication apparatus.
The DAA of the communication apparatus includes a voltage detection circuit. A voltage of the established telephone line is detected by the voltage detection circuit, and a control for regulating the detected voltage is performed such that a preliminarily determined voltage-current characteristics is obtained. Specifically, when the off-hook or a pressing of the speakerphone key is detected, the DAA regulates current on the basis of the voltage detected by the voltage detection circuit to match impedance on the communication apparatus side to impedance (for example, 600Ω) of the telephone line. Therefore, the line is terminated with the matched impedance, so that the communication apparatus can perform good data communication through the telephone line (see JP-A-2005-57659).